[edit] I think you didn't understand what I meant by forks. Every altcoin that is based on the bitcoin client is a fork. I didn't mean forking of the bitcoin blockchain (we could debate on whether the continuation of a fork of the blockchain should also be called an altcoin)
Exponential growth and the S-curve. It needs to be sustained to work.
Bitcoin has aspects of a pyramid scheme. By definition if it succeeds then those that move in last are the ones that lose/profit the least.
All you need is some alt that offers enough incentive to gain strong traction and hold it.
If the profits there are higher than in bitcoin your greed comes to bite you.
Just a hypothetical example: ZeroCoin offers better privacy and the Bitcoin devs can't / won't implement a zero knowledge proof for bitcoin
For some reason people are incentivized to desperately want that added privacy (taxes, repression, you get the idea).
All it really takes is for an alt to be as widely used/accepted to become a real danger.
I personally believe that before that happens the protocol would be adapted. But like I said this is predicting the future and therefore a gamble